The invention relates to a method for reducing frequency crosstalk during the transmission and/or storage of acoustic signals.
The more it proves possible to reduce the data quantity to be transmitted without any noticeable quality loss, the more attractive is the transmission and/or storage of digitized acoustic signals. Numerous methods are known for data compression of acoustic signals by transformation of the digitized data in the frequency range or, a subdivision into different frequency bands.
The subdivision into several partial bands can take place in single-stage manner by a filter bank or in multistage manner by the series-connection of two or more filter banks. (A following filter bank can also be replaced by a transformation.) The thus prepared data are compressed to reduce the data quantity utilizing the signal statistics and the psychoacoustics in such a way that, following transmission and inverse transformation of the data, as far as possible the human ear perceives no difference compared with the input signal. In the proposal for the standardization of data compression methods for data audio signals of the International Organization for Standardization, a hybrid filter bank is used to subdivide the signal spectrum into partial bands. The analysis bank in the coder consists of two stages. Firstly the spectrum of the input signal is subdivided into 32 partial bands by a polyphase filter bank, such as is described, for example, by H. J. Nussbaumer, M. Vetterli, "Computationally Efficient QMF Filter Banks", IEEE Proc. ICASSP 1984, p. 11.31-4. Each of these partial bands is then subdivided once again into 12 bands with a following TDAC filter bank. Such a TDAC filter bank is described by J. P. Princen, A. W. Johnson and A. B. Brodley, "Subband/Transform Coding Using Filter Bank Designs Based on Time Domain Aliasing Cancellation", Proc. ICASSP '87, p. 50.1.1-4.
In the publication by S. A. Towns, T. K. Trong, the VSLI design of a subband coder, International Conference on Acoustic Speech and Signal Processing, vol. 2, 1984, New York, 34B 21-24B 24 a comparison takes place of a filter bank having a tree structure and a filter bank having a parallel structure with respect to the design of subband coders in VSLI technology.
A prerequisite for the use of serial separator stages in data compression methods is a good band separation, so that as far as possible each spectral component of the input signal influences only one partial band signal and therefore quantization errors, which occur in the partial bands, influence only the associated frequency range in the output signal.
Therefore the partial band filters of the overall system must have a very high stop band attenuation. The serial arrangement of filter and/or transformation stages gives rise to an inadequate stop band attenuation, however, because in each case several partial bands occur in the transition ranges of preceding separator stages. In the frequency response of the corresponding partial band filter, this leads to a clear rise of the signal outside the pass band.
Therefore the spectral components of the input signal in the corresponding frequency ranges, after passing through the separator stages, influence the partial band signals in the form of crosstalk components. Following the combination of the partial band signals, quantization errors in one of these partial bands correspondingly influence frequency ranges outside the particular partial bands.
In known methods an attempt is made to minimize deterioration of the output signal due to frequency crosstalk by taking account of this effect during coding. However, this is only possible to a limited extent and makes the coding processes more complex.